In the past, there has been extensive description in the patent and other technical literature of electrophoretic migration imaging processes. For example, a description of such processes may be found in U.S. Pat. Nos. 2,758,939 by Sugarman issued Aug. 14, 1956; 2,940,847; 3,100,426, 3,140,175 and 3,143,508, all by Kaprelian; 3,384,565, 3,384,488 and 3,615,558, all by Tulagin et al; 3,384,566 by Clark; and 3,383,993 by Yeh. In addition to the foregoing patent literature directed to conventional photoelectrophoretic migration imaging processes, another type of electrophoretic migration imaging process which advantageously provides for image reversal is described in Groner, U.S. Pat. No. 3,976,485 issued Aug. 24, 1976. This latter process has been termed photoimmobilized electrophoretic recording or PIER.
In general, each of the foregoing electrophoretic migration imaging processes typically employs an imaging composition containing electrostatic charge-bearing photoconductive particles, i.e., electrically photosensitive particles, positioned between two spaced electrodes, one of which may be transparent. To achieve image formation in these processes, the charge-bearing photosensitive particles positioned between the two spaced electrodes, as described above, are subjected to the influence of an electric field and exposed to activating radiation. As a result, the charge-bearing electrically photosensitive particles are caused to migrate electrophoretically to the surface of one or the other of the spaced electrodes, and one obtains an image pattern on the surface of these electrodes. Typically, a negative image is formed on one electrode, and a positive image is formed on the opposite electrode. Image discrimination occurs in the various electrophoretic migration imaging processes as a result of a net change in charge polarity of either the exposed electrically photosensitive particles (in the case of conventional electrophoretic migration imaging) or the unexposed electrically photosensitive particles (in the case of the electrophoretic migration imaging process described in the above-noted Groner patent application) so that the image formed on one electrode surface is composed ideally of electrically photosensitive particles of one charge polarity, either negative or positive polarity, and the image formed on the opposite polarity electrode surface is composed ideally of electrically photosensitive particles having the opposite charge polarity, either positive or negative respectively.
Regardless of the particular electrophoretic migration imaging process employed, the imaging compositions used in these processes typically contain the electrically photosensitive particles dispersed in an electrically insulating carrier vehicle which, at least during the time of image formation, is in liquid form. In addition, these electrophoretic migration imaging compositions may also contain a charge control agent to aid in dispersing and in imparting electrostatic charge and charge control, e.g., uniform charge polarity, to the electrically photosensitive particles of the imaging composition. (See Col. 23 of U.S. Pat. No. 3,976,485 noted above.) Such charge control agents have heretofore been selected from those materials believed to perform a similar function in conventional liquid electrographic developer compositions. A large number of different materials have been found effective as charge control agents in conventional liquid electrographic developers. A partial listing of representative such materials includes the following:
a. heavy metal soaps, e.g., cobalt naphthenate, and organic surface active agents, including non-ionic, anionic, and cationic surface active agents, as described in Beyer, U.S. Pat. No. 3,417,019 issued Dec. 17, 1968;
b. certain vinyl copolymers as described in Machida et. al., U.S. Pat. No. 3,585,140 issued June 15, 1971 which are prepared from at least three different vinyl monomers, at least one vinyl monomer being selected from each of the following groups: (i) alkyl acrylates and methacrylates in which the alkyl has 8 to 19 carbon atoms; (ii) dimethylaminoethyl methacrylate, acrylonitrile, .alpha.-aminoethylacrylic acid, .alpha.-cyanomethylacrylic acid, N,N-diphenylmethacrylamide, dimethylaminoethyl acrylate, methacrylonitrile, and diethylaminomethyl acrylate; (iii) glycidyl methacrylate, benzyl methacrylate, cyclohexyl acrylate, 2-phenoxyethyl acrylate, crotonyl acrylate, 2-phenylethyl methacrylate and 2-phenylethyl acrylate;
c. copolymers as described in Ikeda et. al., U.S. Pat. No. 3,650,738 issued Mar. 21, 1972 which are prepared from certain monomers having olephilic groups and certain monomers having a basic nitrogen atom; and
d. copolymers as described in Stahly and Merrill, U.S. Pat. No. 3,788,995 issued Jan. 29, 1974 containing repeating units derived from monomers having a specified polar group and repeating units derived from monomers soluble in the carrier of the liquid developer, the polar group-containing monomers selected from materials such as sulfoalkyl acrylates and methacrylates, metal salts of sulfoalkyl acrylates and methacrylates, amine salts of sulfoalkyl acrylates and methacrylates, metal salts of acrylic and methacrylic acids, and amine salts of acrylic and methacrylic acids.
In the course of further research and investigation of electrophoretic imaging compositions involving diverse electrically photosensitive particle materials, including a wide variety of electrically photosensitive dyes and pigments, it has unexpectedly been found that certain of the aforementioned charge control agents (a)-(d), although useful in conventional liquid electrographic developer compositions, appear to deleteriously interfere with the electrical or photosensitive response of certain of these dye and pigment materials. For example, in the case of electrophoretic imaging compositions containing electrically photosensitive particles prepared from the cyan colorant material copper phthalocyanine, Colour Index No. 74160 (Such as that available from American Cyanamid under the tradename Cyan Blue GTNF), it has been found that the normal electrophoretic migration imaging capability of these compositions can be completely destroyed by the incorporation therein of certain polymeric charge control agents of the type disclosed in the aforementioned Stahly and Merrill, U.S. Pat. No. 3,788,995, for instance, a copolymer of sulfoalkyl methacrylate, styrene, and lauryl methacrylate. However, for reasons which at present are not at all clear, this same polymeric charge control agent, i.e., a copolymer derived from a sulfoalkyl methacrylate monomer, appears to function as an acceptable charge control agent for electrophoretic imaging compositions identical to that described immediately above, except that the electrically photosensitive particles of these compositions are prepared from a quinacridone magenta colorant similar to that available from Sandoz Corp. under the tradename Sandorin Brilliant Red 5BL.
The fact that certain materials function as acceptable charge control agents for certain electrophoretic imaging compositions but, at the same time, seriously interfere with or completely destroy the imaging characteristics of other electrophoretic imaging compositions poses an especially troublesome problem. For example, in the case where one contemplates employing a series of two or more differently colored monochrome electrophoretic imaging compositions to form a final multicolor image; the above-noted problem suggests that a series of different charge control agents may have to be manufactured to obtain a charge control agent which is compatible with each different monochrome imaging composition. Alternatively, in the case where one contemplates employing a single polychrome or multicolor electrophoretic imaging composition containing a mixture of at least two differently colored electrically photosensitive particles, each of which is primarily photosensitive to visible radiation of a different wavelength; the above-noted problem suggests that it is unlikely that one will be able to find a charge control agent material which is compatible with each of the differently colored electrically photosensitive particulate materials contained therein. This latter situation is of particular concern because many multicolor electrophoretic imaging compositions represent dispersions which exhibit a high degree of instability as evidenced by the non-uniform charge polarity of the electrically photosensitive particles in these compositions and by the difficulty of maintaining the electrically photosensitive particles well dispersed in these compositions. Thus, many multicolor electrophoretic imaging compositions, which exhibit otherwise useful electrophoretic migration imaging characteristics, clearly could be significantly improved if one could find a useful charge control agent that could be employed in these compositions.